Conventionally, in order to obtain stabilized ground necessary for civil engineering construction in rivers, lakes, marshes, harbours and the sea where said weak soil layer is at the bottom, it is necessary that said weak soil layer is removed and that sand and/or soil is put there. This requires high skill, and is difficult and time-consuming, and is accordingly costly. Moreover, there would be pollution problems in removing and treating a large amount of removed weak soil layer on land.
With those piling methods and/or sand-pile method which are generally known for use in these cases, it is extremely difficult to obtain the stabilized foundation ground necessary for civil construction by driving piles and sand-piles into a deep and wide area of said weak soil layer, and it is also impossible to put these methods in practice as the equipment and/or system necessary for carrying out said methods becomes huge in size.